In blow-molding machines for stretch blow-molding plastic containers, the container is formed from a preform and molded into a container by means of compressed air. Molding a container requires compressed air at pressures of e.g. 25-40 bar. This compressed air is typically generated by means of one or more high-pressure compressors. This compressed air is a major energy consumer in the value chain of a container production process, for instance in addition to polyethylene terephthalate (PET). As energy consciousness is growing, one is looking for ways to reduce air consumption. Various compressed-air recycling systems and special bottle forms are offered by the blow-molding technology in reaction thereto so as to reduce the energy consumption.
However, the question arises whether in the generation of compressed air substantial energy savings can be made. To increase the efficiency of a compressor for providing compressed air, it would in principle be possible to provide a large pressure piston, so that the air used for blowing can be compressed approximately adiabatically. During relaxation the pressure piston would then return to its original position, whereby in strictly arithmetical terms the energy efficiency of the blow-molding process can be increased. However, a problem is the high pressure needed for blow molding. This requires the application of very large forces for the pressure piston, especially in the final phase of the piston compression process. For example, in the case of a piston surface area of 1 dm2, the compression force would be about 40 kN. The application of such a compression force with a corresponding piston at a sufficient piston speed of for example 10 m/s would be very difficult and costly. The air is cyclically compressed by the pressure piston in the prior art. The forces required for air compression air are almost linearly increasing under isothermal aspects. It follows that the force required for compression is not delivered in a constant manner, but peak loads are alternating with empty phases.
DE 10 2009 019 008 A1 shows a method used for blow-molding containers, wherein a preform is shaped, after thermal conditioning, inside a blow mold of a blow-molding machine by influence of blow pressure in the container. At least a portion of the pneumatic pressure energy stored inside the blow-molded container is converted into mechanical drive energy.